Thermite or aluminothermic reactions have been widely used to make high ampacity low impedance electrical connections. There is no better electrical connection. The connection has a current carrying capacity at least equal to that of the conductors welded. The weld makes a permanent molecular bond with the conductors. The reaction materials are contained in a crucible above a tap hole on a fusible disk. When the materials are ignited, the exothermic reaction produces molten metal and slag. The molten metal fuses the disk and runs through the tap hole to a weld chamber to weld any elements exposed to or projecting into the weld chamber. The materials are ignited at the top of the crucible, and the reaction proceeds downwardly to fuse the disk upon a slight delay after ignition. The slag separates from the molten metal rising to the top of the reaction. Any slag drawn through the tap hole forms on top of the weld in the lower end of the tap hole or a special riser chamber. Any slag adhering to the weld is removed after cooling. The crucible, tap hole and weld chamber may be formed in separable refractory molds which are clamped together around the parts being welded and to form the parts of the process noted. The mold parts are cleaned for reuse after each welding process. The reaction materials, the molds and the weld connections made by the process are sold by Erico International Corporation of Solon, Ohio, USA, under the trademark CADWELD.RTM..
CADWELD.RTM. is a registered trademark of ERICO.
The exothermic reaction may be comprised of a reaction between aluminum (Al) and a metal oxide wherein the metal oxide is reduced providing a filler metal, i.e., the source of the filler metal is the oxide on reduction. The "Goldschmidt" reaction is the basis of the application of the process described in U.S. Pat. No. 2,229,045 to Charles A. Caldwell. The reaction is as follows:
Aluminum (Al)+Iron Oxide (Fe.sub.2 O.sub.3)=heat+Aluminum Oxide (Al.sub.2 O.sub.3)+Iron (filler metal) (Fe) or PA1 Aluminum (Al)+Copper Oxide (CuO)=heat+Aluminum Oxide (Al.sub.2 O.sub.3)+Copper (filler metal) (Cu) PA1 Reducing agent+filler metal+metallic compound=heat+weld metal+slag+gases
The "Goldschmidt" reaction has been successfully utilized over the years to weld or join metals such as iron (Fe) and copper (Cu). However, the process is not particularly well adapted for use in joining together a pair of nonferrous metal pieces, such as, two pieces of aluminum (Al) to one another.
U.S. Pat. No. 3,020,610 to Rejdak discloses a method of welding aluminum (Al) and other metals, and provides a listing of various reactions which can be utilized to provide reaction products which may be utilized to provide a weldment. Unfortunately, in the method disclosed by Rejdak the weld which is formed is not suitable for some applications. For example, a weldment produced by the Rejdak method in some environments (such as continuous immersion in rainwater, for example) may be susceptible to a high level of corrosion due to the presence of certain impurities or unwanted elements in the resultant weldment which may adversely affect the life, quality and strength of the weld.
U.S. Pat. No. 2,569,956 to Schiltknecht discloses a process wherein a thermite reaction is utilized to join together a pair of nonferrous metal pieces by melting a sleeve about the metal pieces. This process is usually referred to as the "Alusuisse Process" referring to a welding system produced by Swiss Aluminum Ltd. of Zurich, Switzerland.
U.S. Pat. No. 5,062,903 to Brosnan et al. discloses a process whereby a reducing agent is used to produce heat to melt a filler metal, which then flows out to provide a weldment. Other product components are left behind in the slag with reaction gases released. The process is generally summarized as follows:
The Brosnan et al. invention has been used to weld aluminum metal by using aluminum powder as both the reducing agent and filler metal, and calcium sulfate (CaSO.sub.4) as the metallic compound. The basic reaction is: EQU Al+CaSo.sub.4 =heat+Al.sub.2 O.sub.3 +CaS+Al
The weld metal produced by this reaction was essentially pure (99%) aluminum (Al). However, the reactions were not necessarily uniform and stable and weldments sometimes had lower strengths than that of the aluminum pieces to be welded. The solidified weld metal also had excess porosity in some instances. The process lacked uniform repeatability and stability.
U.S. Pat. No. 5,171,378 to Kovarik et al. discloses an invention which improves on the Brosnan et al. process through the addition of silicon (Si) which strengthens the weld metal considerably. The porosity is also reduced through the addition of sodium chloride (NaCl).
U.S. Pat. No. 5,490,888 to Assel et al. discloses an invention which further improves upon both the Kovarik et al. and Brosnan et al. processes through the addition of two or more metallic compounds which are exothermically reduced by a reducing agent, such as calcium sulfate (CaSO.sub.4). Assel et al. discloses that the reaction mixture preferably contains at least one metallic compound which is a Group I metal sulfate compound such as Li.sub.2 SO.sub.4, Na.sub.2 Sl.sub.4, or K.sub.2 SO.sub.4.
Among the major drawbacks of the Brosnan et al., Kovarik et al. and Assel et al. processes in welding aluminum is that the welds produced by these processes do not possess a suitably high tensile strength, and in the reaction the slag produced by the processes does not necessarily readily uniformly and repeatedly separate from the weld metal thereby sometimes causing contamination of the weld metal and non-uniform results.